1. Field of the Invention
The present invention relates to a vertical oxidation/diffusion processing apparatus.
2. Description of the Related Art
For example, in the manufacture of a semiconductor device, when oxidation/diffusion processing is to be performed to a semiconductor, a vertical oxidation/diffusion processing apparatus having a fused silica processing vessel is used. As the vertical oxidation/diffusion processing apparatus, a processing apparatus having a single tube structure processing vessel (U.S. Pat. No. 5,088.922) and a processing apparatus having a double tube structure processing vessel are conventionally used.
The processing apparatus in which a processing vessel has a single tube structure has the processing vessel of fused silica and a heater arranged around the processing vessel. A gas supply pipe is arranged inside or outside the processing vessel to extend from the lower portion to upper portion of the processing vessel, and a process gas is supplied from the upper portion of the processing vessel. The gas supply pipe is arranged to extend from the lower portion to upper portion of the processing vessel as described above because the process gas is heated to a predetermined temperature by the heater before the process gas is supplied into the processing vessel. When the process gas is heated in advance, desirable oxidation/diffusion processing can be performed. A gas exhaust pipe is arranged at the lower portion of the processing vessel. A wafer boat in which a large number of semiconductor wafers are arranged at predetermined intervals is mounted in the processing vessel, and predetermined oxidation/diffusion processing is performed to the semiconductor wafers.
When oxidation/diffusion processing is performed to the semiconductor wafers by the above apparatus, the temperature in the processing vessel is increased to a predetermined temperature (e.g., 1,000.degree. C.) by the heater. while the semiconductor wafers are placed in the processing vessel, a process gas is supplied from the gas supply pipe. The process gas is heated to a predetermined temperature by a heater while the process gas rises in the gas supply pipe, and the heated process gas is supplied from the gas supply pipe into the processing vessel. The process gas supplied into the processing vessel flows from the upper portion to the lower portion of the processing vessel, and oxidation/diffusion processing is performed to the surfaces of the semiconductor wafers by the process gas. An exhaust gas is externally exhausted from the gas exhaust pipe.
On the other hand, in the processing apparatus in which a processing vessel has a double tube structure, the processing vessel is constituted by an inner tube and an outer tube, a gas supply pipe is arranged at the lower portion of the outer tube, and a gas exhaust pipe is arranged at the lower portion of the inner tube. A process gas supplied from the gas supply pipe rises between the inner tube and the outer tube and heated to a predetermined temperature, and oxidation/diffusion processing is performed to semiconductor wafers by the heated process gas.
However, in the conventional oxidation/diffusion processing apparatuses, the following inconvenience is posed.
When the processing vessel has the single or double tube structure, the process gas is supplied from the upper portion of the processing vessel, flows downward toward the lower portion, and is exhausted from the lower portion of the processing vessel. For this reason, the process gas resides due to the presence of the heat-insulating cylinder arranged at the lower portion of the processing vessel to support a wafer boat and insulate heat, the uniformity of oxidation/diffusion processing of a semiconductor wafer arranged on one side of the lower portion of the processing vessel is poor. In addition, since the gas exhaust pipe is arranged on only one side of the lower portion of the processing vessel, the flow of the process gas flowing downward in the processing vessel is offset on one side. For this reason, the oxidation/diffusion processing of the semiconductor wafer arranged on the lower portion side of the processing vessel is not uniformed.
When the processing vessel has the single tube structure, since a process gas having a relatively low temperature flows in the gas supply pipe, a temperature around the gas supply pipe becomes relatively low, and the temperature of a wafer surface is not uniformed. As a result, the thickness of an oxide film is not uniformed. When a gas supply pipe of fused silica is arranged outside the processing vessel of fused silica and is welded to the vessel, the gas supply pipe is easily damaged and cannot be easily handled at the time of maintenance such as cleaning of the processing vessel. When the processing vessel having the double tube structure is used, its structure is complicated, and the manufacture of the apparatus is cumbersome, thereby increasing the production cost of the apparatus.